A number of hearing protection elements are known which are introduced into the human ear passages in order to prevent or damp health endangering noise. Simple cotton wool is used for this purpose as well as cotton wool fibers which are impregnated with wax-like substances having kneadable properties.
German Offenlegungsschrift [patent application published without examination] No. 2,251,774 discloses a hearing protection element of a foamed polymer material which, due to its special foam structure, can be compressed by the use of pressure and can be introduced into the ear passages in this compressed state. Due to the slow recovery capability of the compressed, foamed polymer, the element, when inserted into the ear passage, will recover its shape after a short time so that the outer contours of the element place themselves against the surfaces of the ear passage and in this way provide a seal which reduces or prevents the penetration of health endangering noise into the interior of the ear passages. This prior publication indicates that any desired, flexible polymer material which can be foamed can be used to produce such an earplug. The essential criterion for this is seen to be that the data given in this prior publication regarding the pressure required for compression and the recovery rate of the foamed polymer must be maintained.
The prior publication employs vinyl chloride plastisol mixtures, foamable polyvinyl chloride plastisol mixtures and polyvinyl chloride mixtures. These are mixed with softeners and expanding agents to produce an appropriately foamed, disc-shaped product out of which are then cut the individual earplugs.
The particular advantage mentioned in the prior publication is that in this way, compressible, plug-like ear protection elements are produced which, due to their compressibility, are adaptable to the size and shape of any human ear passage. The prior publication considers this advantage to be so significant that it states that it is impossible to produce an earplug having a universally applicable size and shape out of a molded elastomer material. Due to their high elasticity, such materials are indicated to be unsuitable for the production of earplugs. In the opinion of the authors of this prior publication, the quick recovery property in particular impairs suitable introduction of the earplug into the ear passage.
The drawback of the prior art embodiments is that the foamable polymers employed, e.g. soft polyvinyl chloride as well as polyurethane etc., contain low molecular softeners, stabilizers or other processing residues which cannot be removed from the finished earplug. These components are not removable and may lead to skin irritations, contact dermatitis or other allergic skin reactions if used as earplugs, i.e. after the earplug has been inserted into a human ear passage. Since the foamable polymer of the prior publication is first brought into a disc shape and the actual plug shape is obtained only afterwards by cutting, stamping or similar mechanical work, there arises the additional drawback that the surface of these plugs is inevitably porous. This inevitably produces a certain chafing effect between the surface of the plug and the surface of the ear passage so that the above-mentioned negative influences on the skin are further supported thereby.
A further drawback is the susceptibility of the foamed polymers employed to the attack of microorganisms, such as fungi, bacteria etc. This susceptibility is known in particular for soft polyvinyl chloride while in connection with the use of polyurethane foams bacterial decay plays a special role. Moreover, at temperatures as they occur in the human ear passages, these damaging fungi, bacteria etc. encounter optimum growth conditions which can again enhance infectious reactions in the ear passage. This effect is further augmented by the porous surfaces of the prior art earplugs as a result of their manufacture and this additionally constitutes a significant contributing factor to easy soiling of the plug during use.
U.S. Pat. No. 4,160,449 discloses an earplug whose reboundable plug material consists of foamed silicone which, for hygienic reasons, is coated with a thin plastic foil. The encasing in a plastic foil is necessary because plugs cut out of foamed disc material or cut off from foamed rod material have coarse pores or open cells, at least at the cut portions, and thus may bring about disadvantageous skin irritations or allergic skin reactions.